Changes any pixel that matches color with the color defined by fill. This method is available if Imagick has been compiled against ImageMagick version 6.3.8 or newer.
function opaquePaintImage($color, $replacementColor, $fuzz, $inverse)
{
    $imagick = new \Imagick(realpath("images/BlueScreen.jpg"));

    //Need to be in a format that supports transparency
    $imagick->setimageformat('png');

    $imagick->opaquePaintImage(
        $color, $replacementColor, $fuzz * \Imagick::getQuantum(), $inverse
    );
    //Not required, but helps tidy up left over pixels
    $imagick->despeckleimage();

    header("Content-Type: image/png");
    echo $imagick->getImageBlob();
}